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How hazardous is it if an engine cowl falls off?
There was recently an incident where the engine cowl of the starboard engine of a United Airline's Boeing 777 fell off during flight:

'Scariest Flight of My Life.' United Airlines Made an Emergency Landing After an Engine Cover Flew Off
Engine Cover Blows Off on United Airlines Flight
United Airlines passenger jet's engine cover rips apart over Pacific Ocean

All available articles seem to suggest it was 'sort' of an emergency and they did in fact do an emergency landing...

A United Airlines flight from San Francisco to Honolulu made an
  emergency landing Tuesday after an engine cover reportedly flew off
  mid-flight.

... with people even asked to brace for impact:

“They let us know that we had to brace for impact in case there was a
  rough landing,” she said.

It seems logical that the cowl coming off could be a sign of loss in structural integrity around the engine (also of the engine itself?).
Another issue could be ingestion of debris (especially near the engine inlet).
How hazardous is such a case of the engine cowl falling off?
Note: I do understand that a twin engine aircraft can continue flying after a single engine loss however due to the reduced performance an emergency landing would make sense.

It would not be good if it hit the tail, but once it's fallen off, the only issue I see is that reverse thrust cannot be used on that engine, it would have been shutdown and just windmilling. But I can't see that really qualifies as an "emergency" landing...? (Or does that mean reverse thrust cannot be used at all due to asymmetry)?

"How hazardous.." I believe it's like in other circumstances where the exact cause and consequences are unknown: The only possible decision is to land in emergency. Further ground analysis will tell how hazardous it has been, from just an engine failure (safe) to the possibility of the engine falling in a turn and hurting the wing or the fuselage (most critical).

Huh!  Would they have shut down that engine ??

The part that fell off is called the engine cowl, the nacelle is the complete structure under the wing including the engine itself.

@Fattie like many of the news agencies you seem to be missing the fact that the engine also lost two fan blades, which is the most likely cause of the loss of casing.

That's not a nacelle.  That's a cover.   The nacelle basically is the engine, and when that departs, it often wrecks so much stuff it renders the plane unflyable.  Here's another where TPHTF...

@Harper The nacelle is the cover. The engine is inside the nacelle. What's missing in this photo is the entire front part of the nacelle, including the cowl.

Well, it's not great...

Regarding the discussion of nomenclature, geographic linguistic differences notwithstanding, the parts in question are parts of the engine cowl (which comprises part of the engine nacelle as a whole). Engine covers refer to "remove-before-flight" type protection equipment such as canvas covers. That being said, in the Media likes to use simple English to describe the cowl as a "cover" of the engine, which is it's function: the cowl covers the engine as part of the nacelle.

Your question implies/infers that the aircraft merely shed a few non-critical parts. That's not a very accurate description of the event. Based on the missing fan blades, a more likely description is failure of at least one fan blade leading to collateral damage, including loss of the cowling. It's a pretty violent event - see https://www.youtube.com/watch?v=wcALjMJbAvU and https://www.youtube.com/watch?v=l8jgGoTc1Fc . That renders the engine inoperable, and for a 777 leaves you with only one good engine.

Whenever an engine fails in flight, there's no way to know how much collateral damage may have been done. Fuel leaks and uncontrollable fires become a distinct possibility. Best thing a crew can do is deem it an emergency and put the plane on the ground before a manageable situation turns into a disaster.

@Harper What do you mean by TPHTF? Google search returns this question and a book reference for "Two-Phase Heat Transfer Fluid", which I doubt is what you mean.

@Skyler I mean The two-phase heat transfer fluid Hits The Fan. Except in this case it's believed to be solid waste discharge from the lav.  I'll let you sit with that thought for a minute.  So perhaps another word might go there.  A four letter word I can't say here.  A word often said when an engine falls off.

Here is an extensive interview with the captain of the flight, discussing the various issues: https://www.youtube.com/watch?v=J7_lzeY23dI

I work in aero engine safety but not for the company that manufactured the engine in the event you mention.
Parts falling off engines is Not Good. We worry about them hitting people below the aircraft. And as Koyovis mentions, losing the nacelle or portions thereof (looks like in this case it was the intake and fan cowling) can change the aeroelastic properties, thus inducing vibration, which is not comfortable for the passengers or crew (reducing their ability to do their jobs) and adds additional loads to other engine parts.
It is not true as mins states that the cause or consequences are unknown. The regulatory authorities require manufacturers to submit a detailed (thousands of pages long) Failure mode, effects, and criticality analysis (FMECA) that covers every part of the engine, what could go wrong, what the consequences could be, and what we are doing to make sure it won't go wrong or that the effects are minimized if they do. While I don't work on that engine, I am confident that structural failure and release of the intake is in the FMECA.
In my professional judgement, in the scale of failures affecting engines defined by the FAA (catastrophic, hazardous, major, minor), this failure would be major, due to a large part falling off the aircraft able to hit a person below, the reduction in safety margins caused by increased vibration, and the damage to critical parts (fan blade) from debris ingestion as mentioned in one of the other answers.
Since the fan blade was seriously damaged, the pilots likely shut down the engine (otherwise the vibration would have been insanely high). But as you mention a single engine shut down is not considered hazardous - we design for that situation. Nevertheless many pilots choose to land after a single engine shutdown.

One of the reasons of landing after a single engine shutdown is to prevent a dual engine shutdown. Which would be Bad.

Mast, agreed. Though we also calculate the probability of that happening, and they are very low (as required by the regulator)

Nice answer! Welcome to Av.se, and thanks for the insider’s perspective on that. I wasn’t aware of the FMECA or how thorough the analysis there has to be. Good stuff!

"It is not true as mins states that the cause or consequences are unknown": Do you mean the pilots knew before landing what were the causes of the accident and what have been the exact impacts on the aircraft? I didn't look after this event, but I'd be surprised. If you mean defect/failure modes analysis during design, then yes, but that's not my point (and I guess nobody predicted the actual impacts of this Trent-900 accident during the design).

I didn't work on the Trent 900 either, but it is definitely true that every time an oil pipe is mentioned in the FMECA, the risk of oil fire causing turbine overheat causing disc burst causing uncontained debris is mentioned. That particular cause of the oil pipe failure was likely not in the FMECA (since the FMECA considers as-designed parts and it was a quality issue, and I believe an unanticipated one at that)

There's that one British Airways flight to London. Taking off at LAX, they lost an engine just past V1.  They continued the takeoff and were able to land... at Manchester.

@Mast - ...which is the old joke. Pilot comes on the PA multiple times saying, "An engine has failed, but we still have X engines left. Landing will be delayed by another 30 minutes.". When they are down to one engine, passenger says, "I sure hope that last engine doesn't go, or we'll be up here all night!"

@Harper Do you know what flight you're referring to, as a full SEI flight from LAX you MAN is implausible.

@TomMcW yes, it's a 2-step process, first use a Boeing, and second, be British Airways.  Seriously  I used to think it was an urban legend but there are better sources than Snopes... The thing actually got investigated by AIB and FAA.   The verdict was the same as the 707 aileron roll... "look, just don't do it again".

@Harper Ahh. 747. SEI on a quad is a bit different. They do SEI ferry flights on those routinely, but with proper fuel planning. An impromptu SEI revenue flight... with passengers... over an ocean? That's pretty risky imho

@TomMcW Yeah, I thought it was a good idea up until about Halifax.  A 3-engine 747 should be considered to have, like "ETOPS 15".  I was figuring BA would have another 747 fly from London or NYC and meet it.

How familiar with an FMECA would a typical pilot be? My guess is awareness of some basic failure modes at most, given the length you mention.

I doubt pilots are aware of the FMECA. If a mitigation requires a pilot to do something, that will be in the engine manual and the operating procedures. The FMECA will clearly state what manual has it. And we try to avoid relying on pilots to fix our problems.

@moink Is there any textbook on how you actually estimate probabilities for these types of failures? I’m a physicist and know probability theory but have no idea bow I’d apply it to engine parts.

@user19997744 there are handbooks for various components. for example for electronics the DoD uses MIL-HDBK-217F: RELIABILITY PREDICTION OF ELECTRONIC EQUIPMENT

@user1997744 There are lots of books on reliability engineering. You can also check out reliawiki: http://reliawiki.org/index.php/Main_Page For each part, we use either a simple #failures/hour or #failures/cycle, or fit a Weibull distribution to previous failures, or a lognormal crack-initiation/crack-propagation model for critical parts. We maintain models of these for engines in service and update them as service data comes in. For new products, we model it based on service data from similar parts in similar engines, or we use rig tests, engine tests, or supplier reliability data.

Aviation is very safe due to the huge experience built up over the decades. Engine failures, as in the engine stops working, are accounted for in flight training and in aircraft design. Safety is in known circumstances, with very low tolerance for situations out of the norm as @corsiKa states.
But an engine cowling falling off is a structural failure, with unpredictable consequences:

What bit of the cowling breaks off?
What other parts of the structure do the broken off bits impact upon?
Are any of the engine cowling parts being sucked into the engines? In this case, it looks like they were.

A typical Level D Full Flight Simulator has about 300 failures which are part of the training curriculum of pilots. These are failures which can be handled by a determined procedure, as experience has shown. About fifty of these are engine malfunctions:

Engine starting: hot and hung starts, start valve failures, ignition failures.
Engine functioning: seizure, vibration, flameout, surge.
Systems: oil pressure, oil temperature, oil filter bypass, oil leak, fuel filter bypass, EEC normal mode, false EGT indication, thrust lever signal fail.
Thrust reverser: unlocked in flight, does not unlock after landing, lock sensors fail.

Note that by far most of these failures are of an engine function that stops working or does not start up when commanded. Only the vibration malfunction may be relevant to a structural failure like a fan blade broken off, but the vibration effects can be neatly calculated with physical modelling, in a predictable and determined way.
Due to the unpredictable nature of the engine cowling structural failure, there is a high degree of uncertainty as to what can and what cannot happen further once the cowling has fallen off; there are bits of structure flapping in the wind, there are vibrations caused by...what? The pilots can see failure effects on the instrumentation, but can be unaware of the exact situation. In the case of El Al 1862:

At 6:28:45 pm, the captain reported: "El Al 1862, lost number three and number four engine, number three and number four engine." ATC and the flight crew did not yet grasp the severity of the situation. Although the flight crew knew they had lost power from the engines, they did not see that the engines had completely broken off and that the wing had been damaged. The outboard engine on the wing of a 747 is visible from the cockpit only with difficulty and the inboard engine on the wing is not visible at all. Given the choices that the captain and crew made following the loss of engine power, the Dutch parliamentary inquiry commission that later studied the crash concluded that the crew did not know that both engines had broken away from the right wing.

Declaring an emergency in the case of the United B777 is fully justified. They are not falling out of the sky yet, but nobody knows for how long that will last...

It's also safe because of a very low tolerance for activity out of the norm. Sure no one wants to be delayed because of the 2000 something safety checks before takeoff, one of those checks failed. I've even heard passengers say things like "I don't know why they bother, things rarely happen anyway" which just baffles the mind - they don't happen because of the zealous safety regulations.

"But an engine cowling falling off is unique." That's simply not true.

My biggest concern would be parts being sucked into the compressor and then the lack of a proper intake, so the engine will operate in very off-design conditions. Thrust will be lower and SFC higher, so the planned destination might not be reached.

@Koyovis, Aviation Herald reports an average of three cowlings falling off a year.  On an aviation scale, that's a fairly common occurrence.

@Mark They break off in unpredictable ways and with unpredictable consequences though.

@corsiKa: Right, things rarely happen because they bother. Such passengers are, frankly, stoopid.

It depends a bit on when it happens. For instance, American Airlines 191 suffered a complete engine loss (it literally fell off) during takeoff. It caused significant hydraulic damage and, due to retraction of the slats on the left wing, caused the airplane to roll over and crash. It's possible the same incident at a higher altitude would have been more recoverable.
The United incident happened at cruising altitude, and only involved the outer cover. The immediate threat is that the engine ingested debris. From pictures of the front, you can see it completely destroyed one fan blade and may have partially damaged another (the pilot likely shut it down, assuming the computer didn't automatically). The vibration in the airframe was likely a result of the loss of the aerodynamic parts in an asymmetric way (problematic but not life-threatening).

Ironically, this happening on a route to Hawaii meant the aircraft was ETOPS certified, meaning it is more fault tolerant for an engine failure like this.

The airplane-engine combination should maintain a target IFSD (In Flight Shut Down, defined as "When an engine ceases to function (when the airplane is airborne) and is shut down, however briefly, whether self induced, flightcrew initiated or caused by an external influence.") rate at or below 0.02 per 1,000 engine-hours for 180min ETOPS approval.

Your answer assumes that the nacelle broke first, debris was ingested and then caused the loss of a fan blade.  While that is possible, it is also entirely possible that the fan blade failed first and the resulting vibration caused the nacelle damage.  Fan blade loss causes a LOT of vibration.

I'd argue that the nacelle did not break first because a single fan blade broke at the root (and damaged its neighbor) while the rest are fairly clean. Debris ingestion results in many fan blades having leading edge damage.

Working closely with aerofoils and their manufacturers, I can say it looks a lot more likely the blade broke first and the nacelle damage is from the vibrations. As @user71659  says, there's only two blades with significant damage. The tips of other blades probably rubbed up against the casing as well because of the vibrations.

From the pictures online it looks like not just outer cover but inlets initial geometry that does initial compression was lost. That’s quite serious, see my answer for slightly more info ;)

Even though there is already accepted answer which is good I would like to add my 2. Source: Masters in gas turbine (jet) engines design.
This is extremely serious issue for a flight. The part that “fell off” is responsible for the air intake and initial compression (before it hits the first stage of compressor). Such change in geometry very likely to change intake pre-compression ratio and cause airflow to more turbulent, which is Not good for the first stage. Debris from the damage are guaranteed to damage first stage of compressor which is very dangerous as the first blade disk is the heaviest and therefore easiest to lose the balance and cause further damage to the engine.
The incident like this could potentially cause serious instability in engines rotor and send the first stage compressor disks flying through the cabin.
Thankfully in civil aviation there are many failsafe mechanisms that will automatically shutdown engine when specific conditions are met. Namely in this case I am very sure the engine was automatically shutdown by the critical level of vibration trigger.
Shutting down engine mid-flight in such situation is the only sane way to ensure that the further damage is not done, therefore all aircrafts designed in a way that allows for loss of up to 50% of engines. Thus in civil aviation medium and large capacity aircrafts are equipped with minimum of 2 engines.
However, even though flight could technically continue on remaining engines, single engine failure implies that there is a possible oversight in pre-post-flight checks, and the crew is required to perform emergency landing procedure in the nearest possible airport.
Worth noting that command to “brace for impact” may have nothing to do with real situation as it is standard procedure for emergency landing and does not constitute real possibility of rough landing.


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Is an FAA Pilot License valid in the U.S. if held by a foreigner?
My question is relatively simple.
Suppose I come to the U. S. and get my whole flight training over here. I hold a green card but the academy only used my Venezuelan passport for the enrollment process. Would my licenses be valid here in the United States even though I'm a foreign person?  I'm taking about the validity of my licenses regardless my status (I'm a permanent resident, green card holder)

I don't know, but strongly suspect that the answer to this would depend on your legal status in the USA, and might possibly depend on your country of citizenship and the type of license involved. To begin with, what kind of visa would you be on while training?

Well. I'm a US permanent resident, I'm just worried because in order to do the whole enrollment process, the academy used my passport (which is from Venezuela) but not my green card...

Yes. Tens of thousands of foreigners - both inside and outside the US - hold FAA certificates, including me. Your nationality is only relevant in these situations:

Foreigners must have TSA approval before starting flight training (US citizens need the approval before soloing). See this question.
Getting the TSA approval might be more difficult for people from certain countries (this is mainly a security/political issue, not an aviation one)
An FAA certificate isn't an immigration document: it doesn't give you any right to enter the US, or to work here. That means, even if you hold an FAA commercial or ATP certificate, you still need a work visa or green card in order to work as a pilot in the US. See this question.

But once you get an FAA certificate, your nationality is irrelevant. The FAA only cares about your license being "valid" in the strictly aviation sense: is your medical current, is your flight review current, is your instrument rating current etc.?
Even if you surrender your green card and leave the US permanently, that won't change anything about your FAA certificate (unless maybe you get expelled and the DHS requires the FAA to revoke your certificate on security grounds). You just send them your new address in whatever country you're in, they update their airman database, and that's it.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


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Can I land a helicopter by controlling only power, not angle of attack?
With full RPM on my rotor system, can I land my helicopter using only the throttle? That is, by just reducing power rather than reducing the blades' angle of attack?

Assuming you mean a manual thrust lever with out a constant speed regulator: yes that is possible but not optimal. Best is to keep the rotor blades at optimum speed and reduce AoA of the blades by lowering collective - if you do that the thrust needs to be adjusted otherwise the rotor overspeeds, but it is still better this way than reducing thrust/rotor speed first.
If rotor speed is slowed down first, the blade speed reduces and with the collective at the same setting, the blade AoA will increase. So the safety margin with stall AoA decreases, at a time when it is required most.
Possible - yes. Recommendable - no.

It should be noted that reducing rotor RPM is part of certain stuck-pedal or damaged tail rotor emergency landings. You should always be wary of turning your governor off, it may be illegal and very dangerous to do so!

Collective only landing is not possible, Because there is forward velocity, and the flight will get out of trim as soon as the throttle and collective are changed.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Is the Embraer E190-E2 equipped with raked wingtips?
It looks like the Embraer E190-E2 has a raked wingtip, but isn't a raked wingtip only efficient for long-haul aircraft like the Boeing 77W?

Possible duplicate of Is a winglet better than an equal span extension?

Why would an aerodynamic device only be useful for long-haul aircraft?

@egid I believe OP is referring to the aerodynamics advantage vs. weight disadvantage, which is a real concern on larger aircraft with raked wingtips and other winglet-like devices (without saying that this is necessarily a concern on a similar scale with smaller aircraft with smaller, lighter components).


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What does the last 'B' in 'GEnx-2B67B' stand for?
What does the last 'B' in GEnx-2B67B stand for?
I guess it stands for Boeing, but the GEnx-1B series doesn't have it.

Often you can get interesting information on engine models directly from the Type Certificate Data Sheets at http://rgl.faa.gov/ In this case I scanned the GEnx type certification and could not find any difference listed between the -2B67 and -2B67B.  Therefore I'm guessing it's a fairly minor change of some kind.

Based on the below (in chronological order), it very much looks like the (or a) difference between GEnx-2B67 and GEnx-2B67B is a booster anti-ice (BAI) air duct. And this suggests that the 'B' stands for 'BAI' or 'booster anti-ice'.

We propose to adopt a new airworthiness directive (AD) for all General Electric Company (GE) model GEnx-2B67 and GEnx-2B67B turbofan engines with booster anti-ice (BAI) air duct, part number (P/N) 2469M32G01, and support bracket, P/N 2469M46G01, installed.

https://www.federalregister.gov/documents/2013/04/11/2013-08447/airworthiness-directives-general-electric-company-turbofan-engines

The Boeing Company and GE asked that the GEnx-2B67 turbofan engine be removed from the Applicability section of this AD. The commenters noted that this engine does not have BAI hardware, therefore, inspection and modification of the BAI does not apply.

https://www.federalregister.gov/documents/2013/08/19/2013-20097/airworthiness-directives-general-electric-company-turbofan-engines
NB: Another hypothesis, that the 'B' is some kind of alphabetic numbering, seems refuted by the lack of an 'A' or 'C' model. Google will find very, very few (1 or 2) hits for 'D' and 'E', which could be typos.


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How do paraglider controls work?
I recently got into paragliding, but I am quite confused on how their controls work.
I am quite aware of how the control surfaces work on fixed-wing aircraft. Let's use for example an aileron. Lowering the aileron, increases the angle of attack of this wing, increasing lift, making the aircraft to roll.
On paragliders, things seem very confusing for me.
The theory
The control acts as a "brake". By pulling one brake there is an asymmetry in drag, causing the wing to yaw. Due to the yawing motion a difference in airspeed in the two sections of the wing causes a difference in lift, having as a side-effect the aircraft to roll too.
Thus pulling the right brake, makes the aircraft yaw to the right, and then roll to the right.
My thoughts
While one of the controls is pulled, the trailing edge gets lowered. This also lowers the aft-portion of the wing cord, increasing the angle of attack. I would expect that this would also increase the lift of this portion of the wing causing the to raise.
Thus pulling the right brake, makes the aircraft roll to the left.
For me, there seems to be nothing special with brakes. For example, in case of emergency (brake line failure), I can use the C-risers to alter the angle of attack of one part of the wing to turn. But then again the same paradox happens. I pull the right C-riser, the right wing has increased angle of attack, but still it dives!
How exactly do paraglider "brakes" work? What makes them behave as a brake, and not as an aileron?

Moving the aileron doesn’t change AoA. Downward aileron increases the camber and upward aileron works as a spoiler, slowing air flow over the wing.

Related answer that addresses the paraglider issue (see below).  To understand the relation between paraglider stability and paraglider control, you have to understand that an uncommanded turn involves some sideslip, and an intentional roll input also causes some sideslip. https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-apply-to-hang-gliders-or-any-aircraft/56548#56548 .

I think the answers so far neglect the curvature of the paraglider sail. Changing the lift at the tips creates an imbalance of the side force caused by the downward pointing tips. This side force will pull the sail sideways and cause the sail-pilot combination to roll.
To illustrate my point, I shamelessly copied this picture and added vectors perpendicular to the sail surface, like this:

Now you will see why I used a photo: The tips of the sail are almost vertical so the side force caused by pulling the local trailing edge down has a sizeable lever arm with the center of gravity (which I assume to be near the pilot's head). @Ken's graphical representation hides this detail and fails to show the lever arm.
This side force will do two things:

It pulls the sail sideways so it will shift away from a position vertical over the center of gravity.
It causes the whole sail-pilot-arrangement to roll, in this case clockwise (from the perspective of the observer).

Next, the increased drag on the pulled-down side will retard this part of the sail, adding a yawing moment and yawing rotation which will now stabilize the bank angle. The whole paraglider turns. With the banked paraglider the side force component of the lift force compensates the centrifugal acceleration (rotating system of reference) rsp. creates the centripetal acceleration (fixed system of reference) which keeps the yaw rotation alive.
Also here the rolling motion is started by a lateral shift of the lift vector, but the vertical component of lift counteracts the desired rolling motion. Only the lateral force, which dominates due to the inclination of the sail, is responsible for the rolling moment.
Exiting the turn is achieved by doing the same, only in reverse.
Now for pitch control: Pulling the trailing edge down on both sides increases camber and lift, so the paraglider rises and the added drag retards the sail. This causes a pitch-up motion around the center of gravity and slows the paraglider down further. Relieving tension on the risers reduces camber, lift and drag so the sail accelerates and the whole paraglider pitches down around its center of gravity.
Therefore, any rotation is around the center of gravity. This does not need any hypothetical pendulum movement or "pendulum effect", which does not exist and is a fallacy anyways.
Oh, and one last word for the believers in the cult of the pendulum effect: I don't care that you downvote what you don't understand. Please try and argue logically and while observing the laws of physics. This, however, will involve the clear danger that you will lose your faith. Apologies for any loss of direction in your lives. That direction was wrong, anyway.

OK, how about considering another mechanism?  Pilot and parachute steady state flight (same velocity), pilot pulls left line, left side slows, pilot continues forward, right wing (initially less lift from lower AOA, now more lift from greater speed), paraglider rolls and pivots around draggier left wing.  The trick is to make the dragger a poor lifter to avoid the initial opposite roll.  Almost a spoiler?  Cheers, from the "cult".

Very nice description of one version of the "pendulum effect" in action.  Sideforce acting high above the CG will certainly create a roll torque.  In hang gliders (and high wing airplanes) something similar exists, but has more to do with the wing's drag vector than a true sideforce vector.  Anyway, nice explanation, and welcome to the cult.

@quietflyer: Oh, come on! A side force high over the cg: That is also what happens when a rudder causes a rolling moment. Same physics, again no pendulum involved. What you write would result in: The F-104 has anhedral because of the pendulum effect. See how ridiculous that line of thinking is?

Wait, you are saying that you think when people talk about the "pendulum effect", they are imagining some sort of actual swinging pendulum like out of Edgar Allen Poe?  That never occurred to me.  Really, I'd rather call it the "keel weight effect" because that makes it more clear that no part of the aircraft is actually swinging relative to other parts of the aircraft.  The only thing is, bouyancy can create a roll torque about a keel weight even with no sideslip- so that's not such a great analogy after all.

Good point though about the tall tail of F-104 causing a dihedral-like pendulum effect in sideslip, so that the aircraft tends to roll in the "downwind" direction..  You meant tail, not rudder, right?  Because the rudder itself actually creates the opposite effect- a roll torque in the "upwind" direction during a rudder-commanded sideslip.  An "anti-pendulum" effect.

@quietflyer: I give up.

Please, don't give up!  The F-104 reflects great progress in the area of pitch control, but actually would have been better with a dihedral wing and a lower tail.  The next one to come along, the F4 Phantom, remedied the Ttail/anhedral disaster.  But the "Zipper" sure was fast.  See the Heinkel 178 (A few people were still around from the teens)

@PeterKämpf I feel your frustration. Please don't give up, who else will fight the pendulum and winglet cult?

@quietflyer I'm 90% certain you are trolling at this point, you keep twisting the meaning of pendulum to mean who knows what. " dihedral-like pendulum effect in sideslip"? Please.

@AEhere -- no, no trolling and no twisting.  Wikipedia says this about pendulum effect-- are they trolling too?  https://en.wikipedia.org/wiki/Keel_effect .  Now, do you have any problem with this statement "Good point though about the tall tail of F-104 causing a dihedral-like pendulum effect in sideslip, so that the aircraft tends to roll in the "downwind" direction"?  Because obviously, if the CG is low, then side area above the CG WILL indeed create a dihedral-like, downwind roll torque during a sideslip.

See also this answer-- did I cleverly plant this answer into Stack Exchange 8 months ago to help facilitate my trolling?  https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-apply-to-hang-gliders-or-any-aircraft/56548#56548

@quietflyer Wikipedia has been known to be outright wrong, so I invite you to check the history of that article and see the same arguments over meaning, lack of citations of any major literature in the field and overall simplistic reasoning without any analytical backup. Also a number of contributors challenging the use of the pendulum wording in literature, I know I have never encountered it in my career.  "dihedral-like pendulum effect in sideslip" so a dihedral effect then? Or simple lateral stability due to tail volume, coupled with roll as it often is? Where's the pendulum?

@AEhere --Sure, a dihedral effect. I guess it never occurred to me that anyone would take seriously the idea that the "pendulum effect" was actually due to gravity pulling down on the CG as the aircraft "pivots" around the center of lift or center of lateral area.  I thought we all agreed on what the "pendulum effect" really was and were just debating how significant it was-- at least one participant is claiming the CG is always too near the center of lateral area for the "pendulum effect " to be noticeable.

@AEhere -- often overlooked is the fact that the drag vector, acting above the CG, can create a roll torque during sideslip even if true sideforce is minimal.

@AEhere -- to me a true dihedral effect is based on a difference in angle-of-attack between two surfaces.  A sphere with a heavy weight on the bottom couldn't experience a true dihedral effect when struck by a sudden side-gust, but it could experience a dihedral-like effect due to the drag vector acting above the CG.

Which leads to a better understanding of CG and aerodynamic forces.  Peter's writing very cleverly illustrates the "brake" is more to the vertical, creating a roll into the turn. Notice the torque does indeed act around the CG to roll in the direction of the intended turn because the "pendulum" (simply a center of mass away from the aerodynamic force) rotates.  If the aerodynamic force were applied closer to the CG, it would be pushed.  Pendulum is really saying aerodynamic forces (including lift) are not acting directly at CG, and I hope yaw creating "antipendulum" roll is not shunned.

@PeterKämpf is the photograph your own work - did you create it on your own, or did you find it somewhere else? I ask because I'm looking for a tool to great similar graphics.

@PeterKämpf excellent! Exactly the answer that I was looking for. Thank you.

@MarkJonesJr. Please follow the link ("this picture") – this is what I started with.

The curve is not massively relevant as square skydiving wings and steerable paraglider reserves work in the same way with brakes.

While one of the controls is pulled, the trailing edge gets lowered. This also lowers the aft-portion of the wing cord, increasing the angle of attack. I would expect that this would also increase the lift of this portion of the wing causing the to raise.

That is correct.
But with the lift also the drag raises. This causes the parachute  to turn into the direction where the control is pulled.
Since the pilots body does not want to turn (yet) it "swings" out in the former direction forcing the other side of the parachute to raise over the side to turn to despite the lift is less here...

But still... Shouldn't the "braked" wing raise, at least briefly and slightly, if there is more lift? – user3634713

Yes, but it is outweighed by the force of the pilots "swinging body" into the other direction. This is because while "swinging out" the pilots weight drags more on the inner "wing" (where the turn is heading to) and the "outer wing"  the parachute is less supporting, therefore the ratio of supported weight to lift gets better then on the "outer wing".

Also why is this not experienced in other aircraft with conventional ailerons? What is different in paragliders? – user3634713

A convertional aircraft has its "reference point" (where all the forces virtually impact) is at most at the same level (in height) where the wings are mounted. Therefore "swing effects" do not matter that much.
A para gliders "reference point" is more than one wingspan below the "wing level".

But still... Shouldn't the "braked" wing raise, at least briefly and slightly, if there is more lift? Also why is this not experienced in other aircraft with conventional ailerons? What is different in paragliders?

The main difference in my mind is the flexibility of the paraglider. The weight of the pilot can pull differently on different strings while the pilot swings around. In an aircraft the pilot position would be fixed and the "strings" could also be loaded in compression.

@PeterKämpf: I think the main difference is the distance of the "reference point" and the actual "turning center" when the object changes the bank angle. On an regular plane (even a Trike) they are close together, on a paraglider they are far away from each other which causes the swing effects and therefore changing forces on the "strings" supporting the pilot.

Of course, the correct way to turn a paraglider is to weightshift in the direction you want to turn before you pull down the brake...

@DaveGremlin to do that you'd have to pull at least half of the strings (if not all) but you pull only a few...

@TimothyTruckle - No, no.  There's no pulling involved.  The standard technique for turning a paraglider is to first look in the direction you are turning to ensure it is clear, second shift your weight in your harness in that direction, finally tighten the turn with the brake on that side.  You can actually turn a paraglider with both arms folded in front of you and just leaning into the turn - I'm sure we've all done it for our instructors during training

I think this answer is missing something.  The pilot's body won't "want" to swing to the outside (due to apparent centrifugal force" unless the turn rate is tightening, and the turn rate can only tighten if a real aerodynamic (centripetal) force is acting to make that happen.  This would be the sideforce from the air hitting the side area of the wing, after the brake input (assuming the turn is executed with brakes alone) makes the wing skid (yaw) to point toward the inside of the turn.  As per last paragraph of https://aviation.stackexchange.com/a/77996/34686 .

So instead of saying the pilot tends to swing to the outside (due to "centrifugal force"?) (or because "it does not want to turn yet"), would be equally valid to note that after the brake deployment makes the glider skid (yaw) to point in the intended direction of turn, the force from the sideways relative wind impacting the wing, acting high above the CG of the whole system, creates a roll torque.  As per last paragraph of https://aviation.stackexchange.com/a/77996/34686

Other content related to the idea that the forces we "feel" in flight are simply the real aerodynamic forces generated by the aircraft and nothing more-- https://aviation.stackexchange.com/questions/77275/forces-felt-by-pilot-g-meter-inclinometer-are-they-the-aerodynamic-forces-g

@quietflyer looks like you're missing the point of the question...

I've also recently started paragliding, and with some background in aerodynamics have convinced myself that the reason paragliders experience this counter-intuitive reaction (right turn from what would be called a left aileron deflection), is that the direction of the lift change is radial from the total vehicle CG, which is about at the pilots chest.  Expanding on some of the comments above, imagine the lifting surface as an arc of constant distance from the CG (I realize that's not completely true, especially for high performance gliders, but it's close), with outward lift components distributed throughout its span, aligned radially from the CG.  If you increase the magnitude of the lift components on the right side, due to right trailing edge pulled down by a right brake input, they still are pointing directly away from the pilot/CG, so they apply NO NET ROLL MOMENT relative to the CG.  The lift components of an airplane wing are essentially tangential, i.e. normal to their arm from the CG, so in addition to generating lift due to right aileron down deflection, they also generate a lot of left roll moment.  The attached cartoon shows why the primary aileron roll effect on a conventional aircraft is negligible on a paraglider, leaving the drag/yaw effect to do most of the work.  The yaw moment yaws the glider to the right, which develops left sideslip.  This sideslip causes a right force on the wing, which now has a long arm above the CG, and rolls the glider to the right, beginning our pendulum-style turn.

This answer has some good insights, but needs some improvement.  Note that the arrow representing net left, weight, and net sideforce are all drawn as if they are acting at the CG or in line with the CG.  This is not invalid, but to get insight as to what torques are generated, it might be better to draw them where they are effectively acting.  In the airplane case, right hand diagram, the lift arrow would be displaced to the right.  In the paraglider case, the lift arrow would be displaced to the right.  Also the sideforce arrowr should be drawn up near this displaced lift arrow, making it...

Also the sideforce arrow should be drawn higher, up near the (displaced to right) lift arrow, so that the vector sum of both is an arrow pointing roughly in the direction of "One o'clock", emanating from the portion of the wing that is roughly in the direction of "One o'clock" from the pilot's body.(Roughly?)

The key insight that is hidden in this answer, but has not yet been fully developed, is that the arrow representing the vector sum of vertical lift and sideforce-- i.e. the arrow representing the net force generated by the wing-- when drawn emanating from the point on the canopy where it is effectively acting-- cannot simply be radial to the CG of the whole system.  Because that would generate no roll torque.  To generate a roll torque, that force vector must be tilted over to point further to the right of the direction that is radial to the CG of the whole system.

You can't describe paraglider control by thinking of the pilot/paraglider combination as a rigid body which rotates around its CoG, as in a rigid-wing aircraft or glider. Nor is it true that the suspension lines apply a radial force through the CoG; if this were the case, the pilot's body would not rotate at all. A paraglider is a compound of pilot, harness and wing, each element able to move to some extent relative to the others, constrained by their flexible links, the most important of which are the carabiners where the risers transmit wing forces to the harness. The height of these above the pilot's body CoG provides the lever arm which enables the wing to cause the pilot's body to pitch. The lateral separation of the carabiners allows the wing to make the pilot roll and yaw; it also transmits the pilot's weightshift to the wing. This is why the chest strap adjustment makes a difference to how the wing flies and to its curvature; and why the presence of a seat plate has an effect. Significant relative rotations occur at this joint; when you come out of a dynamic manouevre, and are going up with the wing behind you, you slow until you start moving downward and the wing shoots forward. If you don't catch it with a brake action at the right moment, it may rotate so far forward (around the carabiner axis) that the angle of attack goes negative, it collapses, and you fall into it. None of this could happen if the entire wing/harness/pilot combination were a rigid body with no relative movements. This is also why thinking of the combination as a pendulum in which the pilot is the weight and the wing's centre of lift, the pivot, is misleading. The pilot/harness may swing around the carabiners like a pendulum to some extent, but it's nothing like a clock pendulum with a fixed pivot.
With all this in mind, the primary effect of pulling one brake is to increase both lift and drag on that side. As the wingtip is curved down, these pull the wing to that side, both rolling and yawing the wing, and the pilot's body follows because of the carabiner leverage. Better technique is to weightshift, increasing wing loading on that side, which has the same effect. Balancing the two inputs gives the most efficient turn.

Interesting answer.  Thought experiment: re "As the wingtip is curved down, these pull the wing to that side, both rolling and yawing the wing, "-- ok, focussing specifically on the "rolling" part-- a) would the same roll torque exist if we deflected an aileron downward (imagine aileron on other side stays fixed) on an airplane that was basically conventional in design, except for extreme anhedral (i.e. whole wing is inclined downward on each side)?  b) what if same extreme-anhedral wing w/ same single down-deflected aileron is mounted on a rigid pylon high above CG of aircraft as a whole?

I understand that your whole point is that the paraglider is not a simple rigid system, but still seems useful to compare and contrast actual paraglider with simplified models such as these

Pulling one brake creates a lateral component of force on the wing (as explained by P Kampf, although I'm not sure about the exact line of the force that he's drawn), while the pilot's inertia resists this movement from below. This starts the wing rolling. The yaw created by the drag on the braked side may also lead to a small sideslip which reinforces the roll, but I'm guessing about whether that is significant. PG wings can't sideslip much without the into-wind tip collapsing. Your rigid aircraft a) is missing the suspended mass so would probably not behave the same way.

I think by pulling down gently on one side the pilot affects a chain of behaviors that cause the turn.
1- pulling the brake on one side, say right side, causes reduction in lift on that side even thought it adds to angle of attak, for it forces that side to function more like a parachute then a wing, with immediate increase in drag and more importantly loweres the same side giving better unbalance lift to the other side.
2- Then because of the extra lift on the left side it swings the pilot up and to the left causing the whole parachute to bank left and turn.
Anybody who has had a heavy bike knows that if you want to turn left you push the left side of the handle forward, or pull right side in, totally counter intuitive.

I think the plus in drag right (pulling right) has more effect than increasing lift on the right side.
Hm, but an airplane would do it too.
I think the moment of inertia plays a role.
Imagine an Aircraft had a huge mass 10m fixed mounted below. When steering right (left aileron down) It would lift the left wing. But it cant because the huge moment of inertia. So the drag increase of the left aileron would be effective and there will be a yaw movement in the left direction.
Could this be a explanation? Huge Moment of inertia in Paraglider compaired to low moment of inertia of aircraft?

Please provide additional details in your answer. As it's currently written, it's hard to understand your solution.

If you increase the angle of attack at the back of the wing it will indeed be pushed upwards, but that angles the front of the wing downwards in relation. Like a teeter totter.

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text:
Are virtual winglets a viable concept?
Just read this abstract by a Regeneron STS finalist.  Is the idea of a virtual winglet a viable concept? Seems interesting.

Somebody needs to tell West Virginia University to start teaching kids proper English. The word "virtual" does not mean what they think it means. I mean, W. Virginia has a ton of issues with education, SAT's and IQ's, but this is over the top. All they are thinking of is redirection of airflow to provide the same effect on the wing as a winglet would.

Oh, come on. It says "high school" in the title, you can't judge it by the same standards as a college paper.

From Webster: "being such in essence or effect though not formally recognized or admitted".  Doing the job of a winglet although not actually being one physically.  I'd say that could be construed to fit.

@JohnK Not even closed. Second part of that sentence kills your argument. :)

@JuanJimenez explain.

The effect of a winglet is quite recognized and admitted. What the paper proposes is quite real and measurable. Nothing virtual about it. The thing about virtual is that it doesn't exist.

You are applying a definition that only applies in the computer world. There are others.  In the physical world, virtual doesn't mean it doesn't exist.  The virtual refers to the fact that even though the effect is there, the physical object, or concept, nominally associated with that effect is of a radically different form.  Like this device.  It's not a physical winglet, but has the same effect as a winglet (in theory).  It's virtually a winglet.  Thay guy over there is a virtual dictator.  He exists; he is not officially a dictator, but he exercises powers that have the same result.

Aston Martin do something similar with their 'Aeroblade', which replaces the rear spoiler, so the concept of using air to guide surrounding air is viable. Whether it's more efficient than using a physical object, remains to be seen.

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When was the first rudder on an aircraft used?
I have seen pictures of World War I-era airplanes that have rudders much smaller than today. When did the first rudder come out?

One point worth keeping in kind is that those WW1 airplanes w/ small rudders often had no fixed vertical tail acting in opposition to the rudder.

Planes as early as the wright glider (1902) had a rudder, it appears from pictures that the 1900 and 1901 variants did not have rudders but its hard to tell from the photos.

Depending on your definition if you are talking about powered flight the Wright Flyer (1903) had 2 rudders much like its earlier glider predecessor.

In the lighter than air category, the Giffard Dirigible (1852) had a sail like rudder in the aft of the ship. So that may take the cake if you have consider it a true rudder.

Side Note: Patented in 1930 the V-Tail design could be argued as the first design that did not (as far as I know) employ a rudder. The earlier Beechcraft Bonanza's are the most common plane of that design in commercial production. There is quite a bit of debate as to the safety of ruddervators and the general theme seems to be that they are not perfect and do require some care to fly properly.

The Wright photos here are misleading: the first shows the 1902 glider with fixed rudders.  They switched to (single, then double) move-able (coupled to the wing warping) rudders later in 1902.  The second photo shows a Model A in 1908.

@aml What's a "fixed rudder"? I thought that rudders had to be movable in order to accomplish their function of allowing the pilot to command a yaw.

@TannerSwett - the Wrights wanted a 'bank & yank' design, but had adverse yaw.  They tried a fixed vertical stab but it didn't work well enough.  Orville conceived of making it move-able, and Wilbur designed roll coupled cables (rather than a separate yaw control).  Back then, 'rudder' could apply to any appendage that gave directional stability or control.  (Additional fins would later be added to distribute the stability [lacking a fuselage to act as a keel] to prevent weather-vaning.)

"Rudder could apply to any appendage..." In German it still does.  The primary control surfaces are called Höhenruder, Seitenruder, Querruder.

@TannerSwett -- I don't recall what term the Wrights used in their writings at the time to refer to their fixed vertical fins, before they made them movable, but that's what "aml" is referring to here.

@aml -- re the "additional fins" part of the comment-- if you know specific places in their writings where the Wrights explain the rationale for these surfaces, consider answering this related question -- https://aviation.stackexchange.com/questions/78013/why-did-the-wright-brothers-take-steps-to-increase-the-amount-of-vertical-surfac

Rudder itself was used before the Wright. It was just the idea of a rudder on a boat. Otto Lilienthal flew a glider which had a tail with a modern look in 1894:

Source
Actually the whole airplane has a modern look, like the cambered wing, Lilienthal was an aerodynamicist, he invented the polar curve.
Note: The rudder is not meant to control the aircraft direction --this is done by rolling the wing,-- but to counteract the adverse yaw which develops when the wing is rolled (more drag on the upper half of the wing). This effect can be obtained even with a fixed rudder (the turn will not be coordinated, but this tends to prevent a spiral dive).

Adverse yaw in a roll (source, more in this answer)
So the size of the rudder is proportional to this force, which is smaller on small wings.

The fixed vertical surface can be traced back to arrows – they are around since the stone age. The movable vertical surface can also be traced back to antiquity – all ships needed those. The movable, centerline-mounted rudder on ships was invented in the Middle Ages. That should be the most likely answer. All hydrodynamic experience was later transferred to aerodynamics.

@PeterKämpf, "[stone age] should be the most likely answer": This question is explicitly related to aircraft only, not to arrows or ships, so that wouldn't be a valid answer. However I weighted talking about Clément Ader, but that was a bit controversial.

1785,
when Jean-Pierre Blanchard

crossed the English Channel in a balloon equipped with flapping wings for propulsion and a birdlike tail for steering

according to pages 26-27 of L. Winter and G. Denger's "Minute Epics of Flight," published 1933.
We don't have photos, but we do have a woodcut.

The Wright Flyer had two rudders.  That flew in 1903.  Some of the wright bros. earlier gliders also had rudders ... To test the technology and to determine the optimum size.

The 1785 balloon answer is interesting.  However, we might note that flapping wings would be extremely ineffective at propelling a balloon, and a rudder will have no significant effect on a free (unpropelled) balloon.  With that in mind, we might conclude that George Cayley was the first to suggest the use of a rudder in a practical way on an aircraft.
In 1799 George Cayley inscribed a coin with a picture of a design for a glider that appears to include a movable rudder.  By 1804 he had built a flying model glider that included a cruciform tail that could be adjusted on the ground both in the pitch and yaw dimensions.  By 1848 he had created an illustration of a full-scale glider that included a similar tail that could be adjusted in flight.  He tested a full-scale glider built along these  lines in 1849, and a larger similar glider in 1853.
Here is one link that covers Cayley's work:  http://www.flyingmachines.org/cayl.html
More recently, the Wright brothers first used a moveable rudder on their 1902 glider, and also used one on their famous 1903 airplane, generally recognized as the first airplane capable of sustained, controlled flight.
Having said all that, I wouldn't be surprised if someone located an earlier example of a rudder in a drawing or even an operational (small) model airplane or glider.  It is a fairly intuitive concept, deriving directly from naval technology-- in fact, the naval precedents might well have tended to lead early inventors to overestimate the advantages and results of having a movable rudder on an airplane.

Yes, the 1785 "oars" and rudder were nautically inspired and must have been entirely ineffective.  But the rudder was at least intended to work and, for all we know, they thought that it worked.  This wouldn't be aviation's last failed experiment by any means!


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Why has the folding wing option of the Boeing 777 never been ordered?
The Boeing 777 has an optional folding wing mechanism to fit into tighter spaces, but it has never been ordered by airlines, according to Wikipedia. Why is this option so unpopular?
Now with the Boeing 777X, Boeing is again offering a smaller, less complex wing folding option. Has this been ordered, and are other airliner models planned to incorporate folding wingtips?

Ethan, please consider passing by in chat before asking questions, your posts keep being flagged by the system as short and incomplete. You seem to have the right amount of curiosity, but if you don't learn how to ask questions it will be difficult for others to give you answers.

@Federico I usually dont ask question Iin the chat thats just me. My questions may have negative results but I still get the answers I need. I personally think the chatroom is more for asking for definitions. And didn't you ask a question in chat didnt get the answer and had to ask the question on the main site.

Ethan, I am suggesting you ask help in chat on how to properly ask the questions on the site, I am not suggesting to ask the questions only in chat.

It's a nice question thought from Ethan, but I feel like my upvote rep should go to Peter for putting in the effort here.

The folding wingtips are found mostly in naval aviation. The reason is the space constraint inside the hangar of the aircraft carrier.

"US Navy 070128-N-9712C-003 Aircraft fill the hanger bays on board USS Ronald Reagan (CVN 76) after recently embarking Carrier Air Wing One Four (CVW- 14)" by U.S. Navy Photo by Mass Communication Specialist 2nd Class John P. Curtis.
As can be seen from the photo, folding the wings offers the only practical way of fitting fighter aircraft inside the cramped hangar space. For example, the F/A-18C/D had a wing span of 11.43m; this was reduced to 8.38m with the wings folded, a 26% reduction.
However, the price to pay is an increase in complexity, weight, price and increased maintenance. The figure shows the wing folding mechanism of a F 18.

Source:polepositionimagery.com.au
The major drawbacks of folding wings are,

The increased weight and complexity due to the folding mechanism. The folding mechanism takes time to operate and there are safety concerns if they get stuck in a position.
The folding of wings limits the fuel capacity and as a result, the range of the aircraft, as fuel is usually not carried on the folding part of the wings.

Source: zarco-macross.wikidot.com
As can be seen, the F/A doesn't carry fuel in the folding part of the wing. In fact, after the airlines didn't take up its folding concept in 777, Boeing added fuel tanks to that area and increased range of the 777-200/300.
There has previously been no reason for airliners to fold their wings, as airports offered enough space for operations and ground movement. However, as the size of aircraft and their aspect ratios grow (to reduce drag), designers are finding it more and more difficult to keep the wingspan below the 80m 'box' which airlines believe is the maximum they can cope up economically.
The first major program to be affected by this limitation was the A380, which limited the wingspan of the aircraft to less than 80m and reduced its aspect ratio.

"Giant planes comparison" by Clem Tillier (clem AT tillier.net) - Original Work. Licensed under CC BY-SA 2.5 via Commons.
Basically, Boeing is facing the same problem. The proposed 777x is the largest-wingspan aircraft Boeing has ever produced. However, in the case of the A380, Airbus managed to reduce induced drag through the use of wingtip fences, among other design decisions. The 777x is expected to have a raked wingtip, which is not as space effective as the wingtip fence.

Source: Flightglobal.com
As a result, Boeing is considering folding wingtips. However, there are important differences between the original plan in 777 and the proposed plan in 777x.
In the original plan, Boeing proposed to fold 21' of the wingtip. However,the new plans call for folding only 10'.

Original Wing folding plan in Boeing 777. Source:www.seattletimes.com

Image:Boeing
As can be seen from the figure, the folding plan is much more modest. This is not expected to have the drawbacks associated with the the original plan, as,

The end of the wing will not house controls.
As the size of the wing to be folded is smaller, the folding mechanism will not be as heavy and complicated.
That part probably will not carry fuel.
Advances in actuation systems since the original 777 will reduce the weight penalty.

To put things into perspective, the 747-400's winglets were 6' feet tall.
Airlines have already placed orders for 777x and it is their operational record that will determine if other aviation manufacturers (Airbus, that is.) will follow suit.

why cant this apply to all airliners?

It can
It can apply to any new or redesigned airliner. It's probably not something you'd need, want or be economically feasible to retrofit to existing in-service airliners.
Disadvantages
Whether the manufacturer or operators want the associated disadvantages is another matter.

additional expense,
complexity,
weight and
safety/maintenance burden

Reliability is critical. A 777X may be unable to vacate a runway if a wingtip fails to fold, and it has to do this in a short time (of the order of 20 seconds).

Should the wingtips not fold up and lock into place correctly after the aircraft lands, it could effectively paralyze an airport by delaying both departures and arrivals

- Liebherr-Aerospace senior executive Heiko Lütjens
It's just the tips
Note that only the tips fold, otherwise there would be many more problems in catering for hydraulic, electrical, structural and fuel systems.
Competition
The 777X has a wingspan of 71m. Only when folded are it's wings about the same width, 64-65m, as its competitor, the A350-1000.
Airport gate width
Wingspan matters when designing an aircraft to operate into existing terminal gates at the specific airports used by the makers' customers.
Boeing wants their 777X to be able to fit where the old 777 and its newer competitors can fit.
Aircraft    Wingspan   Passengers
747-400     60m        345†
777-300     61m        299†
A350-1000   65m        369
777X        71m        350‡
A380-800    80m        469†

† Most of the passenger numbers are from British Airways for 4-class operation. They don't represent anywhere near max capacity but are probably closer to typical?
‡ Typical 3-class operation.
Airbus had to put serious effort in getting airports to accommodate the A380. I guess Boeing really doesn't want the 777X to be competing with the higher-capacity superheavies for space at the bigger gates.

if the tech already exists

The notion of folding wings has been around a long time. Since before 1913

It has mostly been used on small aircraft that operate from facilities where space is severely limited.

Boeing's ideas in this area have been known since at least 2006

For completeness I would add a small description of the drawbacks (weight, complexity, fuel storage/pipes) to explain why it is not that common.

@Federico: Thanks, I've made that more explicit.

All of Otto Lilienthal's gliders had folding wings (backwards, not up) so they could fit through the standard railway wagon door.

Note that the part about 'effectively paralyzing an airport' would only apply if the taxiways were not approved for ADG VI aircraft. Any airport that can handle a 748 or A380 would not be affected. They just wouldn't be able to park the plane in an ADG V stand. For airports that have not been designed to support ADG VI aircraft, they'd most likely bring out ops vehicles to help ensure wingtip clearance during the taxi. They might also need to not use an adjacent runway or taxiway during the taxi. It's unlikely that it would actually be forced to just sit on the runway for long, though.

The B777x share most of the characteristic of B777. However, while B777 is Code E(ICAO)/Group V(FAA), due to b777x wingspan, it falls into Code F(ICAO)/Group VI(FAA) catalog.
Currently only A380 and b748 is in  Code F/Group VI catalog, which the fleet size is limited (317+123), only a few airliners has such planes and not many airport can parking place and other infrastructures for those plan.
When B777x provides wing folding option, it can feet the code E/Group V parking place. The B777x can be served for most of the airport and reduce the cost.

Wing fold is not new idea.It appears after WWI, when aircraft carrier has been developed, because of the limited hangar size, most of the naval aircraft are designed as wing folded.
E-2 Hawkeye with wings folded:

F-18C with wings folded:


text:
Why isn't the wing span of a certain airliner longer with the longer fuselage versions?
The Boeing 737 next generation models (700,800,900ER) all have the same wingspan at 117ft 5 in. Why doesn't the wingspan increase with bigger and longer models? With the 737-900 wouldn't you need a longer wingspan than the 737-700 because of the extra weight you would be carrying and you need more lift to support that extra weight.

Short answer: They only need to develop, build and maintain one type of wing. And there's no reason not to do it.

That, however, leads to the question: How does it affect performance?
From the formula for aerodynamic lift we see there are generally 4 ways to increase a wing's lift:

Increase the angle of attack
Increase the wing area
Increase speed
Increase air density

Angle of attack
The angle of attack is what defines our aircraft's state, like stall, best cruise, etc.. So we can take it as fixed for a given situation.
Wing area
As you suggested, we could increase the wing's span or depth.
Speed
Lift goes with speed squared. To cope with the additional weight of the larger aircraft, we need to increase airspeed for a given situation by the square root of the load factor. Let's say the larger aircraft is 1.5 times as heavy, all speeds scale with  sqrt(1.5) = 1.22. So all characteristic speeds like stall speed, best climb speed, best cruise speed, etc. 1.22-fold.
Air density
Well, we can't change air density. We can, however, fly in denser air, i.e. lower. To put it the other way round, the heavier aircraft suffers from a lower service ceiling.
Conclusion
Given the same wing, the heavier aircraft performs worse in terms of service ceiling, short take of and landing, but better when it comes to cruising fast.

Why must a 102ft-long 737-600 fly at the same angle of attack in all situations as a 138ft-long 737-900? Cruising at a higher AoA would be one way of generating the extra lift needed by the longer, heavier body. Your observations about speed are thwarted by the fact that the smaller 737-600 has a slightly higher cruise speed than the heavier -900. All 737NG models have the same service ceiling (41,000ft). And your conclusion is backwards: you've argued that the heavier aircraft needs to cruise faster, but you conclude that it's better at going faster. Except, in reality, it goes slower.

@DavidRicherby Characteristics like stall speed and most efficient cruise speed are determined by the angle of attack, so I fixed the AoA and observed how these characteristics change. You sure can fly a -900 at the -600's best cruise speed with a higher AoA then. Do you compare maximum or most efficient cruise speeds? I was always talking about "best cruise" as: the speed where range is maximized. That is actually higher for the -900 (at a given altitude), although that maximum range is lower for the -900.

@DavidRicherby The efficiency (fuel/weight) is the same for both aircraft at the same AoA, so the heavier aircraft can fly faster without lowering efficiency. If the maximum speed is limited by engine power, the -600 can go faster because it has a higher thrust to weigth ratio, sacrificing efficiency. It's easy to see that if both aircraft fly at the same power, the -600 a bit faster but the -900 with a lot more payload, the latter is more efficient.

@DavidRicherby Service ceiling is limited by the coffin corner. As you climb, you must compensate the thinner air with speed or AoA, but you can't increase the AoA beyond stall, so it's mostly speed. The heavier aircraft has a higher stall speed and as stall speed approaches the speed of sound as you climb, the heavier aircraft will reach that point sooner and therefor have a lower service ceiling. Note that all of the above also valid for empty vs. loaded aircraft, so the reason why Boeing gives the same service ceiling for all aircraft may be that it depends more on loading than model.

@DavidRicherby The 737 family's 41000ft service ceiling is actually the max. altitude the pressure cabin is certified for. See [http://www.b737.org.uk/pressurisation.htm#reduced-cabin-alt-sys] and [http://www.airliners.net/aircraft-data/stats.main?id=96].

There are two main reasons:

It is easier to plug in a fuselage section than a (swept back) wing section. If you notice, one thing that has been kept constant across models is the fuselage cross section. Basically, the manufacturer can make another fuselage cross section and add it to the exiting ones.
If a wing section is added, on the other hand, the aircraft characteristics will change appreciably, in addition to the difficulty in doing so. If you are adding wing section, quite a lot of additional modifications have to be made (like control surfaces, etc.) which will increase cost, eliminating any cost savings the manufacturer could've had by using common parts across models.

The next, perhaps more important reason for this from airline viewpoint is that it will change the aircraft design group. The following table is from the FAA AC 150/5300-13 Airport Design (The ICAO definitions are more or less the same):

Image from FAA AC 150/5300-13 Airport Design
For example, the Boeing 737 and Airbus A320 have wingspans at the limit of the design group III (B737 has 117 ft 10 in while A320 has 117 ft 5 in with sharklets). Any increase will change the ADG, which will restrict the number of airports that the aircraft can be operated from, which the airlines won't like.
For example, when the 747-8 wingspan was increased, the aircraft category was changed (from ICAO category E to F, equivalent to V to VI) and compatibility studies had to be carried out to permit operations in airports were 747-400s are already in operation.
The airlines are perfectly happy to use different operational procedures or sacrifice some other characteristics (like range- for e.g. B737 MAX range drops around 200 miles between MAX 7 and MAX 9; aircraft rarely operate those distances anyway) for having the same aircraft operate in a large number of airports and reduce maintenance burden by having common parts across models.

+1 for the ADG. 737 can't increase wingspan and still be able to park in the spots designed for it. Hence the use of winglets.

A wing is very hard to expand, unlike a fuselage when it has a cylindrical mid section.
Wings need to be dimensioned to not break when loaded with the maximum conceivable air loads in operation. These are reached when the aircraft flies into a strong updraft at high speed, and speed limits must be obeyed in gusty weather to make sure the wings stay intact.
If some area is added at the tips, it will create a lot of bending moment (lift force times lever arm) for relatively little lift. This bending moment is responsible for the stresses at the wing root and must, therefore, be limited so the wing will not break off.
In order to produce more lift, only the flaps are modified. Compare the two pictures below: The upper shows the flaps of an A319 and the lower shows those of an A321:

A319 flaps in landing configuration (picture source)

A321 flaps in approach configuration (picture source)
While the A319 flap is single-slotted, the A321 uses a second flap segment with an additional slot to eke out a little more lift from the same planform. It also has a deeper flap, so wing chord is slightly increased. This helps to make the landing speeds of both aircraft similar, while the maximum landing weight of the A321 is 27.5% higher than that of the A319. For similar take-off performance, more powerful engines are selected, but for landing performance the wing needs to create more lift. In cruise the wing will simply be operated at a higher lift coefficient.
Only for the A340-500/600 series was the wing area increased by inserting a triangular mid section into the airfoil, so both chord and span could be increased over that of the A340-200/300. This was possible due to the geometry of modern supercritical airfoils, but is a very rare occurrence. Besides allowing the wing to create more lift, this expanded the internal fuel volume by 50%, so the A340-500 became the world's longest range airliner when it was introduced in 2002. It also increased the root structure and its maximum bending moment, so the wing span could be extended.

A340 version comparison (own work). Blue is the A340-200, red is the A340-500. Note the wing chord increase while the tail surfaces remained unchanged.

The flaps are modified to increase lift for takeoff and approach. How do they increase lift in cruise to account for the extra weight? Do you just increase cruise mach or does it fly at a higher aoa?

@TomMcW: In cruise the wing will simply be operated at a higher lift coefficient. This is done by flying at a higher angle of attack.

or a lower altitude, I should have added.

One point that hasn't been mentioned yet:
The fully loaded and empty weights of an aircraft can differ greatly. For a typical airliner the takeoff weight can be between about 25% and 45% fuel https://en.wikipedia.org/wiki/Fuel_fraction
Therefore it is perfectly possible for a long fuselage variant used on short haul routes and a short fuselage variant on used on long haul routes (requiring a lot more fuel) to have exactly the same takeoff weight, and therefore wing loading.
So yes, a given wing design has a maximum allowable loading, but the way that can be distributed between % plane + % fuel + % payload can vary significantly.
On the other hand, it's certainly true that due to standardization, there are many planes out there with overdesigned wings. You can see the A380 wings have quite a large area compared with the rest of the plane. They were designed with the idea of stretching the plane later, but there hasn't been much interest in a stretched version yet.

With the 737-900 wouldn't you need a longer wingspan than the 737-700 because of the extra weight you would be carrying and you need more lift to support that extra weight.

You would, if the 737-700's wingspan was designed at the design limits for the 737-700's fuselage. But it isn't. Instead, the entire family's wingspan is designed at the design limits for the largest member of the family.
In other words, we could rephrase your question as:

Aren't the 737-900's wings too small?

And the answer is: no, the 737-700's wings are too large!

It is correct that more lift is required to lift the extra weight of the larger variant. This could be achieved with a bigger wing, but also by flying faster or at a higher angle of attack or both.
The whole point of introducing aircraft families like the 737 lies in covering a big market segment with as many common aircraft parts as possible (i.e. reducing development and stock cost and realising savings through commonality). Changing the wing for each model would negate that purpose, as the wing is one of the more expensive parts to be designed on an aircraft. Additionally, it would make in-service maintenance and repair more burdensome, as fleet commonality would be reduced.


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How does a boeing 707 have 4 engines instead of 2?
I was reading a article Link It stated that lower bypass ratio jet engines have higher thrust than high bypass ratio engines, but the boeing 707 has 4 of these low bypass ratio engines and only travels about 600mph. Most modern high bypass ratio engines travel at about 514mph(push boeing 737-800 aircraft at an average of this speed) and only have 2 of these high bypass ratio engines  Boeing 737-800 engine.I am having trouble understanding how this works.

There was about a 35-year gap between the 707 and 787; that's a lot of time for engine technology and efficiency to improve in

Another "why A has X engine instead of Y" question

This is too incomplete to be an answer, but using rough numbers from Wikipedia: The 707-320B has a max takeoff weight of 333,600 pounds and has four engines producing 19,000 pounds of thrust each.  The 787-10 has a maximum takeoff weight of 553,000 pounds and has two engines producing 76,000 pounds of thrust each.  Each engine on the 787 produces as much power as all four engines combined on the 707.  So even though the 787 weighs 200,000 pounds more than the 707 and has half as many engines, it actually has more power per pound of aircraft weight.

How? Because if it only had 2 engines, it wouldn't get off the ground. Why? See Brian's comment and Marco's answer.

@Ethan, this is literally an exact clone of your two previous "Why does [airliner] have 4 engines instead of 2?" questions. If you need clarification about this, why not ask for it in an existing question?

@egid Not exact. Then I was asking why the 4 engines now this question askes more about bypass ratios and thrust and why 2 engines have the same amount of thrust as 4. So there is a difference.

Can you try rephrasing it that way, then? The answers are currently reading between the lines to get that question. Currently it very much reads as 'why does this airplane not have fewer engines?'.

Boeing 707 was initially designed to use the P&W JT3C turbojet engine.
Early versions of the '707 (like the one in the first picture of your question) and C-135 (including KC-135A) used those engines.
Wikipedia reports a maximum thrust of 12,030 lbf.
The JT-3D is one of the firsts early low-bypass ratio turbofans (1958/59). It's based on the JT3c and, according to Wikipedia, has a bypass ratio of 1.42:1 and a maximum thrust of about 17,000 lbf.
The CFM-56 first run was in 1974, more than 15 years later. The CFM-56 is a high-bypass ratio turbofan, with a bypass ratio of 5.5:1 and a maximum thrust of 19,500 lbf for the CFM56-7B18 model that powers the Boeing 737-600 as reported on Wikipedia.
So the answer to your question is that at the time the Boeing 707 was designed, the only available engines were turbojets with a limited (for today standards) thrust.
The article you're asking about refers to modern jet engines. Today low-bypass ratio turbofans are used mainly in military supersonic aircrafts, were fuel efficiency is less important than thrust and supersonic efficiency.
The opposite applies on modern civil aircrafts.

Engine technology has improved tremendously since the days of the Boeing 707. Today's engines are larger, more powerful, more efficient and quieter than their predecessors.
The engines used in first production Boeing 707s (from 1958), the Pratt & Whitney JT3C turbojet, developed a thrust of 50KN (11200 lbf). This engine was the first one (in US) to develop more that 10000 lbf. The same engine (Pratt & Whitney J57) was used in the B-52B stratofortress.
This engine was selected as it was the best available (in fact, the J57 designers won the Collier trophy for their efforts). In fact, the engine was so successful that aircrafts were designed around it- half of the century series USAF fighters used them, as did 707's competitor, the Douglas DC-8.
As for Boeing 737, which had its first flight around 10 years later, the Pratt & Whitney JT8D used was a low bypass turbofan engine, having a bypass ratio of 0.96:1. Again, it was developed from a military engine, the Pratt & Whitney J52 (which powers Northrop Grumman EA-6B Prowler, among others).
One pattern we can see clearly is that most of the early civil aircraft engines were developed from military turbojets. The reason was that only these powerplants could generate the required thrust at that time and today's high bypass turbofans were not even in the picture.
However, as the airliners became fuel conscious and noise became a major issue, high bypass turbofans were developed exclusively for civil use and the development of civil and military engines diverged. With time, the bypass ratios have increases with decreases in fuel consumption.

Source: compositesworld.com
As a comparison, the Rolls Royce Trent-1000 engines used in Boeing 787 can generate 350KN (78000 lbf) of thrust, more than 6 times that of JT3C. So, for meeting the thrust requirement of the four engined Boeing 707, you don't even need a single 787 engine.
In the context, it is more useful if you compare the engines used in the same aircraft over the years. The most powerful engine used in the 707, the Pratt & Whitney JT3D-7 increased thrust over JT3C by over 50%. In case of 737, the 737 MAX uses CFM International LEAP, which doubles the thrust on the PW JT8D.
As for the article, what I get is this: For a given power available to the bypass fan, it can be used in either the two ways- take less air and increase the pressure of it more or take more air and have a lesser increase in pressure.
In general, the military turbofans have a lower bypass and higher thrust to weight ratio. For example, the Pratt & Whitney F119 used in F-22 Raptor has a bypass ratio of 0.3:1 and a thrust to weight ratio in excess of 9, while the General Electric GenEx used in Boeing 787 has a bypass ratio of 9.6:1 and a thrust to weight ratio of 5.


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Is it a better idea to put propellers on the wings than the nose?
Most propeller aircraft have either a single propeller at the nose or two propellers on the wings. Nose propellers have a big disadvantage as the big fuselage can obstruct the airflow of the propeller and reduce its thrust. Wing-mounted propellers are only obstructed by the engines (which are much smaller) and the wings. Although the airflow over the wings causes some drag, it also increases the lift, while nose propellers don’t have this advantage. Nose-mounted single engine aircraft are still widely used because internal combustion engines are very big, heavy and expensive. When a single engine is sufficient, it will save a lot of cost compared to twin-engine aircraft.
Electric aircraft don’t have this problem. Electric motors are very small, very cheap and need little maintenance. As a result, it doesn’t cost much to increase the numbers of propellers. Because electric motors are very small, the engine pylons can be made much smaller to minimize the interruption of the airflow. As a result, wing-mounted electric motors should better leverage their size advantage and become the standard solution of electric aircraft. However, many experimental electric aircraft (e.g., Pipistrel Velis Electro, eFlyer 2, and eFusion) still have a single nose propeller instead of two wing propellers. So I am wondering if I overestimated the advantage of wing propellers.

Considering how many aircraft have nose/center-line mounted engines and how many have wing mounted engines, one would be hard pressed to claim that one is a "better idea" than the other. As in all aspects of aircraft design, there are a series of trade-offs made, and in some cases (most notably those with odd numbers of engines), the nose is deemed a highly desirable location to mount an engine.

The chief disadvantage to wing mounted engines and propellers is loss of control due to asymmetric thrust in the event of an engine failure.  If one engine goes, the thrust line is offset from center of mass, resulting in a yawing moment.  At low speeds, the rudder may not be sufficient to counteract this moment, resulting in loss of control of the aircraft ie the dreaded and fatal ‘Vmc roll’ known to multi engine pilots.  Multi engine airplanes have been produced with centerline thrust designs with an engine in the nose driving a tractor propeller and an engine aft of the cabin driving a pusher propeller.  Probably the best known and most successful of these designs is the Cessna model 337 Skymaster.  These arrangements do not suffer from the thrust asymmetry problems described above in the event of an engine failure, but they do result in fuselage space consumed by engines and limitations on the configuration of the design due to structures, controllability and center of mass issues.
You are correct that it’s very easy to mount a distributed electric propulsion system in small pods along the leading edge of the wings, each one housing an electric motor driving a propeller.  Because of the asymmetric thrust issues, this design is not favorable for an initial pilot trainer, which is why designs like the E flyer or Pipistrel‘s Alpha Electro favored a nose mounted motor offering conventional centerline thrust.

Mounting of propellers is strongly related to aircraft function design.
If possible, thrust is generated most efficiently by one large prop.  Drag is also reduced through smaller tail surfaces if the engine and prop are mounted in front, on the nose, which places CG forward.
Once the design limitation of prop arc is reached, the next best thing is 2 on the wings.  Accelerated airflow from wing mounted prop blast add a bit more lift, reducing the required wing area, which saves weight and drag.  Span loading the wing also increases its tolerance to manuvering G forces.
This is why wing mounts were very popular for long range prop driven aircraft, and remain with jet airliners.
One draw back on wing mounts is that a sudden increase in throttle will create more of a nose-up pitching tendency than a nose mount (by way of greater increase of lift).  Mounting the engines with a down angle solves this but results in a loss of efficiency (downward thrust component).  Nose mounts are easier for aircraft such as trainers, where throttle changes are much more common than with long distance cruisers.

As usual in the aerospace world, also the placement of the propeller(s) is the result of compromises among conflicting requirements.
For some of the most common configurations a list of pros and cons is given. Note that some pros of a configuration can be cons of another one and viceversa. Feel free to expand each point.

Front tractor propeller

pros: the propeller works with undisturbed  airflow which gives a better efficiency (note that this is actually the opposite of what is supposed in the question); the engine gets also fresh air for cooling purpose; better accessibility for maintenance.
cons: blocks the front vision; the wake affect the aerodynamic of the whole airplane; noise and vibrations "flow" in the cabin.

Aft pusher propeller

pros: the propeller sucks air in, helping  against flow detachments from the aft fuselage lowering the drag; better visibility; less noise in the cabin.
cons: weight shifted backward reducing the leverarm of the tailplane which has therefore to be bigger giving more drag and weight; propeller less efficient; longer landing gear to avoid collision with the ground during the rotation phase.

Wing mounted

pros: the weight of engine+propeller compensate for the lift relieving the wing's structure which can be lighter.
cons: unbalance in case of OEI (one engine inoperative) which has to be compensated by a bigger vertical tailplane giving more drag and weight; nothing safety-critical (pilot included) should be on the trajectory of a brocken propeller; in case of a low-wing design, longer landing gear are required; mutual aerodynamic interference between wing and propeller (note that also this is actually the opposite of what is supposed in the question).

Also other less common configuration exist, some of them are shown in the next pictures (all pictures Wikimedia).

Burt Rutan's Voyager, a push-pull configuration with a pusher and a tractor propeller, optimised for very low consumption.

Burt Rutan's Boomerang, a double propeller asymmetrical, designed asymmetric to mitigate OEI operation

Over the fuselage propeller, configuration normally used for amphibious airplane to get propeller+engine as far as possible from water (Icon A5 depicted here)

That's for the propeller(s). Obviously the electrical engine has different features/requirements than a heat engine that have to be taken into account as well.


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How to calculate lift force of a revolved NACA 0009 airfoil?
How would I calculate the lift force as a function of angle of attack for a revolved NACA 0009 3D body (incompressible flow)? I have data for CL vs. AoA for the 2D airfoil, but how would I use this to calculate the lift force on the body?
In the lift equation, I'm specifically wondering what to use for CL and S. (dynamic pressure is straightforward)
Would I just use planform area of the 3D body for S and 3D, incompressible lift coefficient for CL? Where AR is the aspect ratio of the body. See below for 3D adjusted equations.

It looks like an airship  maybe this report NACA can help you. Anyway do not expect it to generate lot of lift: I would say an order of magnitude less than the relevant airfoil. And for sure you cannot use the equation you posted which is for wings i.e. something with a span much higher than the chord and the thickness.

Thank you sophit. This paper is excellent

You're welcome

This 3D shape will not generate appreciable lift.  It will not behave like an airfoil.


text:
Authorization of repairing engine blade nicks
Since blade nicks repair considered as a major repair, I would like to ask,
1- Who is authorized to repair engine blade nicks?
2- Could a mechanic with powerplant rating do the repair?
3- Could a sheet metal specialist do the repair ?

From 14 CFR 65.85, emphasis mine:

Except as provided in paragraph (b) of this section, a certificated mechanic with a powerplant rating may approve and return to service a powerplant or propeller or any related part or appliance, after he has performed, supervised, or inspected its maintenance or alteration (excluding major repairs and major alterations). In addition, he may perform the 100-hour inspection required by part 91 of this chapter on a powerplant or propeller, or any part thereof, and approve and return it to service.

So a mechanic with just a powerplant rating cannot approve the airplane for return to service after a major repair. This requires an IA. 14 CFR 65.95:

(a) The holder of an inspection authorization may—

(1) Inspect and approve for return to service any aircraft or related part or appliance (except any aircraft maintained in accordance with a continuous airworthiness program under part 121 of this chapter) after a major repair or major alteration to it in accordance with part 43 [New] of this chapter, if the work was done in accordance with technical data approved by the Administrator

The IA does not need to perform the repair themselves. Anyone working under the proper supervision can do the actual repair, from 14 CFR 43.3(d):

A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the extent necessary to ensure that it is being done properly and if the supervisor is readily available, in person, for consultation. However, this paragraph does not authorize the performance of any inspection required by Part 91 or Part 125 of this chapter or any inspection performed after a major repair or alteration.

The IA needs to inspect the repair afterwards and make sure it conforms to requirements.
The manufacturer is also permitted to perform the repairs, per 14 CFR 43.3(j).


text:
fuselage structure
How can a mechanic measure the distance of underlying structural elements of fuselage skin (e.g. stringers, internal doublers around door cutouts) from a specific cracked fastener hole in the fuselage skin through external inspection, given that SRM schematics don't include all required dimensions?
Is it necessary to recourse to internal inspection subsequent to removing internal interior stuff (side walls, insulation blankets, etc.)?

Please follow the instructions here to get the accounts merged.

the last account is scheduled for deletion

This sounds like a question for whoever is responsible for the SRM content.

Use the inspection panels to look inside.
Follow the fastener locations.
When all else fails, disassemble.


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Does adding twist to wing always decrease induced drag and what are its effect on parasite drag?
I am doing an assignment and one question was the effect of twist on drag. The first two options are increased induced drag and decreased induced drag respectively. The third option is increased parasite drag. I am confused b/w option 2 and 3.

I think what is missing from the question is the frame of reference for what part is fixed, and what part is twisted.  In other words, given an untwisted wing at a given AoA, twisting either the root up or the tip down will have opposite effects.  (You might get brownie points for understanding from your professor if you ask for this clarification)

If you do a bad job twisting a wing, you can certainly make induced drag worse.
Twisting a wing should not have a significant effect on parasite drag.  If you have a very detailed parasite drag model, you might be able to calculate some difference, but to first order, no change.
Twist is also useful for manipulating the stall distribution -- inboard or outboard.  This is essential for handling qualities.
It also shifts the load distribution which can have a significant structural effect.


text:
What are the reliability statistics for modern jet engines?
What is the actual numerical reliability statistics of the modern, large-aircraft, commercial turbofan engines available today?
e.g. How many running hours can we expect a failure after? Are there empirical or projected MTBF statistics?
How is credible MTBF established for a new engine model?
Is there a significant difference in the failure rate between engine models or manufacturers?
I was motivated to ask by this snippet on another answer on SE Aviation:

In the days of the 747 regulations required at least 3 engines if you
  ventured more than 60 mniutes of flying time away from a suitable
  diversion airport (in case the second one also failed). However as
  reliability of engines increased this got extended with ETOPS
  certificates.

How rapidly has engine reliability improved? Do we have a time trend of reliability numbers?
Why do modern airliners have only two engines?

I'm certain this is a dupe, or at least addressed elsewhere, but my query-fu is failing me.

I found a few rough figures here:  http://www.pprune.org/engineers-technicians/441924-gas-turbine-engine-mtbf.html  but more detail might need to be more model-specific...

What is the actual numerical reliability statistics of the modern, large-aircraft, commercial turbofan engines available today?
e.g. How many running hours can we expect a failure after? Are there empirical or projected MTBF statistics?

Buried in this GE press release is the comment that the GE90 has 'only one engine IFSD  per one million engine flight-hours'.
This ICAO presentation has a lot more info, including these threshholds for ETOPS IFSD rates:

Propulsion system should achieve an in-flight shutdown (IFSD) rate of:

0.05 / 1000 engine hours for EDTO 120 min
0.02 / 1000 engine hours for EDTO 180 min
0.01 / 1000 engine hours for EDTO beyond 180 min
Rate computed based on world fleet data, on a 12-month rolling average

This strongly implies that the reliability targets are empirical, not based on theoretical numbers.

Is there a significant difference in the failure rate between engine models or manufacturers?

Page 56 of the presentation above has some data, but IMHO not enough to draw any conclusions.

How rapidly has engine reliability improved? Do we have a time trend of reliability numbers?

All I can say is it's definitely improved massively, but I'm not sure where to find the info.

Welcome to Aviation.SE this answer is more of a link-only answer. It would be better if you elaborated more on it.


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Is sustained inverted flight possible with any of today's commercial jet aircraft?
After viewing the movie " Flight", I was wondering if extended inverted flight was possible with any of the current larger commercial jet aircraft?

Theoretically, it is possible to fly an aircraft upside down. Good examples are the barrel roll done by Alvin "Tex" Johnson in Dash-80 (the predecessor of Boeing 707) and the actions of Pilot Flying James Tucker during the hijacking of Federal Express 705. However, the question is whether the maneuver can be sustained for and reasonable periods of time.
In this matter, the statement of Boeing (manufacturer of MD-80 after merger with McDonnell Douglas) is instructive:

The MD-80 cannot sustain inverted flight,...The MD-80, as with all commercial airliners, was designed to fly upright. Commercial airliners are only tested and certified for upright flight.

The pilots of Alaska Airlines Flight 261, a MD-83, tried to fly their aircraft upside down for recovery, but were unsuccessful.

Note that the barrel roll is not really inverted flight. As far as aerodynamics are concerned, the aircraft is still flying normally. Its lift vector is still upward with respect to the wings at 1 g. This is completely different from actual inverted flight where the lift vector is downward with respect to the wings (i.e. -1 g for sustained inverted flight.)

There's a nice video on youtube of a guy pouring water into a glass in a glider doing loops and barrel rolls. The water always flow into the glass proving that during the "inverted" part of a barrel roll "gravity" points up instead of down therefore the plane isn't flying inverted. (of course, I'm aware that what's pointing up is actually the reaction to the centrepetal force but to the occupants of the plane it might as well be "gravity")